Data suggest that rubisco (ribulose-1,5-bisphosphate carboxylase) makes up 10% of the total protein found in spinach leaves. Research elucidating the structure of rubisco shows that it has four active sites. Why, with four active sites, might there be such a large concentration of rubisco in plant matter?

a.All heterotrophic organisms depend on plants, either directly or indirectly, for their food supply.
b. Rubisco is a very slow enzyme; what it lacks in speed, it makes up in quantity.
c. Three of the four active sites bind carbon dioxide; only one binds ribulose-1,5-bisphosphate.
d. Rubisco catalyzes four of the six reactions found in the Calvin cycle.